Metals, such as copper and tin alloys, can become increasingly hard and inflexible due to stress introduced into the metal from physically shaping of the metal from bending, stamping, coining, punching, flattening, extruding, hammering, stretching and other conventional means of shaping metals. “Working hardening” from the shaping and reshaping of metals can result in metal fatigue ultimately resulting in failure of the metal.
Work hardening is concern in the production and in particular the reloading and reuse of metal cartridge casings. A cartridge casing typically comprises a tubular casing having a closed end and an open end in which the bullet is seated. The metal surrounding the open “mouth” end of the casing must be sufficiently flexible such that mouth can be crimped against the bullet. The mouth must also flex outwardly during firing to seal against the chamber walls of the firearm to create a pressure tight seal for propelling the bullet. The shaping process for forming the tubular casing can harden the metal around the mouth preventing the mouth from properly flexing during loading or firing. In addition, many high power cartridges are necked such that the diameter of the mouth is reduced to less than the diameter of the closed “head” end of casing. The necking process can further harden the mouth of the casing.
The flexing of the mouth and the rest of the casing during firing can also cause work hardening of the casing. As a result, casings that are recovered after firing and reloaded with a new bullet are typically harder than new casings. Moreover, the reloading process typically involves mechanical reshaping and re-crimping of the cartridge casing further hardening the cartridge casing. Similarly, spent cartridges are often reshaped to fit firearms that the cartridge was not originally manufactured to fit. In addition to being performed by individuals with little or no formal training, these reloaded or repurposed “wildcat” cartridges are largely unregulated for safety and effectiveness.
Work hardening of the casing caused by initial manufacture and reloading can harden the mouth of the casing until the casing cannot flex to seal against the projectile or the chamber walls. A work hardened casing could fracture, rupture or otherwise mechanically fail, potentially causing injury to the shooter and/or damaging the firearm.
Annealing is a common technique for counteracting the affects of work hardening that involves softening the metal by heating the metal into critical temperature range at which the stress caused by the shaping of the metal is released. The casing is then cooled with air or water. The casing must carefully be heated to a temperature sufficiently high enough to soften the metal for the stress to be released while less than a temperature where the metal becomes too soft and weaken the structural integrity of the casing.
A related concern is that the metal of the rest of the casing and in particular at the head of the casing is typically harder than the metal at the mouth of the casing. The harder metal at the rear of the casing is necessary to prevent rupturing of the casing during firing and to direct the propellant gases against the bullet and down the barrel of the firearm. The annealing process can apply heat to more of the casing then the mouth portion potentially weakening the rear of the casing. In addition, even if heat is only applied to the portion of the casing to be annealed, the heat can often migrate along the length of the casing even though direct heat was only applied to a portion of the casing.
A currently available technique for annealing the mouth portions of the casings comprises passing the mouth portion of the casing through at least one flame oriented to contact only the mouth portion. The casing is typically positioned on a moving structure such that the mouth of the casing remains in contact with the flame for a limited period of time to prevent heat migration throughout the casing. In addition to the inherent risk of having an opening flame in an ammunition loading area, a drawback of the approach is that the temperature of the flame is typically greater than the preferred annealing temperature creating a danger of overheating the mouth portion of the casing or causing rapid heat migration through the casing. Similarly, an open flame can heat the surrounding air causing convective heating of the casing including the head portion. In addition, flame size, shape, direction, closeness to the mouth portion, and heating time must all be exact to avoid either incomplete annealing or catastrophic case damage. These factors are commonly arrived at empirically, as it is obviously difficult to accurately measure the temperature of the cartridge case along its length in real-time under these conditions.
A related drawback is that a single flame will not evenly heat the case around its entire circumference, leading to uneven metal hardness in the area to be annealed. As a result, the cartridge are commonly rotated within the flame to apply the heat around the entire circumference of the casing. Certain currently available methods utilize multiple flames at different orientations or rotate the casing within the flame area to evenly heat the casing around the mouth portion. Both remedies significantly increase the complexity of the process and the likelihood of failure.
Other approaches to annealing only the mouth portions of the casing include dipping the casing into a molten solution, such as molten metal or chemical salts. However, these approaches typically leave solidified residue on the casing, which can render the case unusable or toxic depending on the molten material used. Similarly, the molten solution itself is often cost prohibitive and can produce toxic fumes. In addition, the solution often requires frequent regeneration or replacement and can often corrode the casing.
Other even less satisfactory annealing processes have been tried, including dipping the cartridge case into molten metal such as lead, or into molten chemical salts intended for specialized heat-treatment and surface-preparation of metals. In the former example, the molten metal tends to wet the surface of the cartridge case brass, leaving behind a solidified residue which makes the case unusable. In the latter example, the salts are usually highly toxic, via either skin contact or the breathing of heated fumes, or both; and many such salts have proven corrosive to the brass cartridge case material. Further, such salts are expensive and have a limited useful life, requiring frequent regeneration or replacement.
As the currently available annealing processes are often dangerous and difficult to control or repeat, there is a need for an effective means of annealing the mouth portions of casing.